High-efficiency solar cells or photo diodes are known to approach theoretical limits in collection efficiency, relating to the short circuit current. However, the actual conversion efficiency is derived by a product of short circuit current, fill factor, and open circuit voltage. It is the open circuit voltage, and to some extent the fill factor, that is known to stubbornly resist all attempts for further improvement to also approximate theoretical limits. It has long been known that recombination is a major factor that limits the actually achievable conversion efficiency. However, practical reasons prevent avoidance of recombination centers in the entire solar cell. Moreover, cross-diffusion of donors and acceptors produces automatically pairs of these that are suspected of being efficient recombination centers.
It would be desirable if some techniques could be developed for increasing the open-circuit voltage of solar cells or photo diodes to levels approaching the maximum attainable collection efficiency for the short circuit current.